How Stars Make Their Own Light

Hold your hand up to the Sun on a clear afternoon and you are catching photons that began their journey not eight minutes ago, as the textbooks sometimes suggest, but tens of thousands of years ago — possibly longer. The eight-minute figure is only the last leg of the trip, the sprint across empty space from the Sun's surface to your skin. The earlier part of the journey, from the core to the surface, is where the real story of starlight lives.

A star is a self-regulating furnace held together by its own weight. In the Sun's core, the pressure of roughly two octillion tonnes of overlying gas crushes hydrogen to densities greater than lead and heats it past fifteen million kelvin. At those conditions, hydrogen nuclei — bare protons, since the electrons have long since been stripped away — occasionally collide hard enough to fuse. This is nuclear fusion: the welding of light atomic nuclei into heavier ones. In the Sun, four protons are converted, through a chain of intermediate steps called the proton-proton chain, into a single helium nucleus.

The helium nucleus that emerges weighs slightly less than the four protons that went in. That missing mass, less than one percent of the total, is released as energy according to Einstein's relation E equals m c squared. Because c, the speed of light, is enormous, even a sliver of mass yields a tremendous amount of energy. The Sun converts roughly four million tonnes of mass into energy every second, and has been doing so for about four and a half billion years without noticeably shrinking.

Most of that energy emerges as gamma rays — the most energetic form of light. If those gamma rays could fly straight out, the Sun would be a lethal X-ray lamp. They cannot. The interior of a star is so dense that a photon travels only a fraction of a millimeter before it slams into a particle and is absorbed. The particle re-emits a photon almost immediately, but in a random direction. The result is a slow, drunken stagger called a random walk. A photon born in the core takes somewhere between ten thousand and a hundred thousand years to reach the surface, losing energy at each collision and shifting from gamma rays toward the visible spectrum we recognize as sunlight.

This slow leak of energy is what holds the star up. Gravity pulls every layer of the Sun inward; the outward pressure of hot gas and trapped radiation pushes back. When these two forces balance throughout the star, the configuration is called hydrostatic equilibrium. It is a remarkably stable arrangement. If the core were to cool slightly, pressure would drop, the core would compress, fusion would speed up, and the temperature would climb back. If it overheated, the core would expand, fusion would slow, and the temperature would fall. The Sun is, in this sense, its own thermostat.

Different stars run this furnace at different settings. A red dwarf, with perhaps a tenth of the Sun's mass, fuses hydrogen so gently it can shine for trillions of years. A massive blue star burns through its hydrogen in a few million, because higher core temperatures accelerate fusion enormously. When a star eventually exhausts the hydrogen in its core, it begins fusing helium into carbon, and — if it is heavy enough — carbon into still heavier elements, all the way up to iron. Iron is where the chain ends; fusing iron consumes energy rather than releasing it, and the star can no longer support itself.

So the light falling on your hand is the end of a long chain. Mass became energy in a proton-proton collision deep in the Sun's core. That energy wandered outward for millennia as a photon scattered countless times among ionized particles. It finally escaped the surface as visible light, crossed ninety-three million miles in eight minutes, and was absorbed by your skin. Stars do not emit light so much as slowly leak the consequences of having squeezed themselves hard enough, for long enough, to fuse their own atoms.